In a cellular wireless wideband system, signals from different base stations may interfere with each other on the same channel or on adjacent channels. A scanner is used to measure the interference of each channel of the wireless system. The scanner utilizes a channel list and a signal type to search possible signals for channels in the list. For each channel, the scanner reports all signals that it detects and measures the power and/or quality of each signal. This process is sometimes referred to as a “Top N” signal scan.
The method is based on the assumption that all channels are uniformly separated in frequency. The current Top N signal scan method is thus best suited to detect interference that occurs across channels including: (a) co-channel interference in which the expected signal and interference signals are on the same channel; (b) adjacent interference in which the expected signal and interference signals are one channel space away; (c) alternative interference in which the expected signal and interference signals are two channel spaces away; and (d) other channel differentiated interference.
With the above definitions, a user can search an interference source by scanning the expected channel and nearby channels sequentially. Typically, a scanning receiver is used in conjunction with a drive test system. The scanning receiver may be configured to automatically and continuously sweep across a pre-selected frequency, plotting signal occupancy within the frequency spectrum being swept.
A typical scanning receiver measures the Top N strongest signals according to a specified wireless protocol. By way of illustration, a Top N procedure may involve the following steps: (1) tune the scanner RF hardware to the carrier frequency of the channel that needs to be measured; (2) down convert the RF signal to baseband and sample the data to memories; (3) according to different wireless protocols, detect the various signals in the collected data; (4) measure the signal power and/or quality of the identified signals and report the results; and (5) tune to the next channel in the channel list. The steps are repeated until all channels in the list have been scanned.
The scanning process for multiple channels thus requires that signal acquisition and processing steps be performed sequentially. Signal data is thus acquired multiple times, thereby increasing processing cycles and the time to produce meaningful results.
The Top N procedure may not be ideally suited for all protocols. In a modern wireless system, such as a system using orthogonal frequency-division multiplexing (OFDM), the assumption that all channels are uniformly separated in frequency cannot be assured. Thus, the modes of potential interference may be more complicated.
For example, in the emerging Long Term Evolution (LTE) standard under consideration by the 3rd Generation Partnership Project (3GPP), the signal bandwidth may be configured from 1.4 MHz to 20 MHz. LTE adopts OFDMA as the downlink channel. The bandwidth of the channel can be configured to 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. If a 1.4 MHz signal has partially overlapped a 20 MHz signal, it is difficult to determine the interference type with conventional Top N scanning because the assumption that channels are uniformly separated is no longer valid.